1. Field of Invention
The present invention relates to a read-only memory (ROM) and the fabrication thereof. More particularly, the present invention relates to an ultraviolet-programmable P-type mask read-only memory (Mask ROM) and a fabrication thereof.
2. Description of Related Art
In the prior art, a P-type Mask ROM usually comprises a lightly doped N-well in a substrate, a bit line in the N-well, a word line perpendicularly crossing over the bit line, an ONO composite layer between the N-well and the word line, and a silicon oxide layer between the bit line and the word line.
However, since the integration of electronic devices always gets higher and the dimensions of electronic devices become smaller and smaller correspondingly, a leakage current easily occurs between the bit lines during the operation of a P-type Mask ROM because of the small line-pitch. Accordingly, a method for fabricating a P-type Mask ROM is provided in the prior art to solve the leakage problem. The method features with performing a blanket erasing implantation after the bit lines and the word lines are formed, so as to increase the dopant concentration of the lightly doped N-well and thereby improve the leakage problem.
However, some problems are encountered in the modified process described above when the dosage of the blanket erasing implantation is poorly controlled. Specifically, when the dosage of the blanket erasing implantation is insufficient, the leakage problem still remains. When the dosage is too high, however, plenty of dopants will diffuse laterally into the channel to cause the Short Channel Effect (SCE).